Adsorption techniques have been used in the separation of a variety of gases including hydrogen, helium, argon, carbon monoxide, carbon dioxide, nitrous oxide, oxygen, and nitrogen. Feed gases useful in these adsorption separations include air; refinery off gases; and land-fill, flue, and natural gases.
Pressure swing adsorption (PSA) systems have been used to produce such gases by the preferential adsorption of impurities contained in the feed gas. PSA systems typically operate in a cyclical process in which beds containing an adsorbent are pressurized to operating pressure with a feed gas, impurities are removed from the feed gas to obtain a product gas, and the beds are regenerated to remove impurities from the system. When multiple beds are used, a pressure equalization step may be employed to equalize the pressure between an exhausted adsorbent bed containing impurities and a regenerated bed substantially free of impurities.
It is likely in the commercial operation of PSA systems that demand for product gas may increase or decrease from time to time. For example, PSA systems are often required to produce product gas at higher rates during normal working hours than during off-hours. Various methods of meeting this variable demand or product turndown have been attempted. These include venting excess product gas, lengthening the cycle time, storing excess product gas, or temporarily shutting down the system.
Wagner, U.S. Pat. No. 3,703,068, discloses a multi-bed PSA system wherein the pressurization rate of successive beds is controlled by the introduction of fluctuations in the product flow and pressure.
The quantity of product gas withdrawn can also be regulated according to product gas withdrawal demand by advancing cycles in successive and parallel cycle systems rather than running them concurrently, as disclosed in Pietrusewsk;, U.S. Pat. No. 4,140,495.
A variable rate compressor pump has also been utilized to correlate product demand to product supply as disclosed in Sebastian et al., U.S. Pat. No. 4,197,096.
Leitgeb, U.S. Pat. No. 4,323,370, discloses varying the length of time of the adsorption phase and the rate of flow of the product gas from the adsorber in response to a varying demand for the product gas. The flow rate and adsorption cycle times are determined as a function of a desired product gas purity and not of the actual product produced by the system.
Product turndown in a multi-bed system has also been regulated by varying the equalization time as described in Armond et al., U.S. Pat. No. 4,576,614.
None of these systems, however, have provided a direct and immediate adjustment of the feed flow rate in response to a change in product demand for a given purity level.
It is known that for a given rate of gas production, a variation in the feed flow rate will cause an undesirable change in the purity of the gas product. This purity drift occurs because a change in the feed rate affects the manner in which the feed gas contacts the adsorbent material, such as by reducing the amount of time the feed gas is in contact with the adsorbent bed. As a result, varying the feed rate will vary the rate of gas production at the expense of the purity of the product.
It is also known that control of the purity level of a product gas can be accomplished in PSA systems by varying the product flow rates at a fixed cycle time. The product flow rate is set at a level greater than that needed by the consumer while the feed gas flow rate either is fixed by throttling or regulated automatically.
For example, purity control in the form of inventory control employing a surge vessel is disclosed in European Patent Publication No. 0 135 921.
Miller et al., U.S. Pat. No. 4,693,730, disclose a product purity control pressure swing adsorption process in which a characteristic of the effluent from concurrent depressurization is sensed, and corrective action is taken in response. Any action can be taken which is effective to vary the impurity concentration in the product gas including adjusting the adsorption time to control the impurity loading of each adsorbent bed, adjusting the concurrent depressurization termination pressure to control the impurity breakthrough at the product end of each adsorbent bed, and/or adjusting the amount of purge gas received from each adsorbent bed to control the extent of regeneration.
European Patent Publication No. 0 207 686 discloses controlling oxygen concentration by controlling variations in the cycle time at which a further adsorption bed is substituted for any exhausted or substantially exhausted bed prior to the purging of the latter bed.
Gunderson, U.S. Pat. No. 4,725,293, discloses a process of controlling purity by small variations in the feed flow rate while allowing product flow to vary at the choice of the consumer.
The undesirable result of these purity control systems is that any substantial change in product demand leads to a temporary change in purity level which can only be corrected by slowly varying the feed flow rate. Because the control system takes too long to respond to a change in product demand, there is a resultant undesirable purity change with a possible loss of product. Thus, prior art PSA systems have suffered from the disadvantage of being able to control product demand or purity but not both.